Thermal drift compensation system and method for optical networks

ABSTRACT

A system and method for detecting and compensating for thermal drift in an optical network in a manner that enables an increased number of optical channels to be used on a given optical medium, such as on a single optical fiber. A pair of narrow band, closely spaced optical signals from an optical transmitter function as a &#34;temperature probe&#34; signal. The two narrow band signals are centered within one passband of a filter of an optical device, such as an optical router. When the two narrow band signals are transmitted back to an optical receiver via the router, the magnitudes of the two signals are compared and a determination can be made as to the magnitude and direction of thermal drift of the passbands of the filter of the optical router. A control subsystem is then used to control a heating/cooling subsystem to either heat or cool the transmitter to induce a shift in the optical signals being generated by the transmitter that causes the transmitted optical signals to effectively track the centers of the passbands of the optical router.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is related to concurrently filed U.S. application Ser.No. ______ (Boeing Docket No. 05-0073) and Ser. No. ______ (BoeingDocket No. 05-0074).

FIELD OF THE INVENTION

The present invention relates to the detection of thermal drift inoptical devices used in optical networks, and more particularly to thedetection, monitoring and correction for thermal drift in opticaldevices employing a plurality of independent optical channels.

BACKGROUND OF THE INVENTION

Electromagnetic interference (EMI) has been a challenge of long standingwith mobile platform, and particularly with aircraft electronic systems.The development of fly-by-wire control systems to reduce vehicle weightand volume increases the risk of EMI. The possible use of EMI weapons todisrupt electronic subsystems used on various forms of mobile platformsand, particularly on commercial and military aircraft, poses anadditional consideration that will likely gain in importance with time.

The use of “fly-by-light” systems would eliminate the risk of EMI tovarious electronic systems used on mobile platforms. However,fly-by-light systems are difficult to build in a form that is bothrobust enough to operate in aerospace environments, and which havesufficient capability of dealing with the larger number of data andcontrols points in a vehicle control network implemented on a mobileplatform, for example, an aircraft. Nevertheless, the use of optictechnologies represents one potential way to reduce the volume and massof the traditionally used integration and control networks implementedon mobile platforms.

A principal obstacle in implementing optics based control networks inaerospace applications has been the somewhat limited number ofindependent optical signals that can be transmitted per optical path(i.e., per optical fiber). Thus, a key consideration in making the useof an optical based control network practical in an aerospaceapplication is the ability to increase the number of wavelength channelsthat can be implemented on each optic path. However, in aerospaceapplications, where various components being controlled by opticalsignals may be exposed to harsh environments and experience significanttemperature changes, thermal drift of the wavelength bands associatedwith the optical channels of a given optical component must beaddressed. If thermal drift could be readily compensated for, then thewavelength bands defining the independent optical channels could beplaced closer together than what would be possible in a thermallyuncompensated for system. This would allow a greater number ofwavelength bands to be used in a given optical medium, for example, on asingle optical fiber.

One approach to controlling thermal drift is employed in thetelecommunications industry where presently up to 64 separate wavelengthdivision multiplexing (WDM) channels can be put on a single opticalfiber. Implementing this number of separate WDM channels requires veryclose control of the wavelengths that define each WDM channel. Thetelecommunication industry's approach to controlling thermal drift is toput all temperature sensitive devices on thermal control units whichcontrol the temperature of the devices to within about 0.1° C. Thosedevices are placed inside temperature control enclosures which controlthe temperature to within about 2.0° C. On land, the enclosures areplaced in temperature control buildings. At sea, the disclosures areplaced at ocean depths of known constant temperature. Obviously, thisdegree of temperature control is impossible and/or impractical toimplement in aerospace applications. In aerospace applications, forcinglarge numbers of optical signals onto one fiber does not produce theoverwhelming cost benefits that it does for telecommunicationapplications. Aerospace applications typically involve fewer signals tosend, over shorter distances, and inside a vehicle. As a result, thecross complexity and mass that would be required to be added into amobile platform, in the form of complex transmitters and receivers usedto put large numbers of signals on single optical fibers, does not giveaerospace applications the same cost savings that are present fortelecommunication applications.

To the contrary, the requirements of aerospace applications can be metby an optical based signal in which relatively modest numbers (i.e.,typically 20 or less) signals are placed on a single optical fiber. Thiswould allow operating wavelengths to be spaced sufficiently far fromeach other and the wavelength bands of the various devices madesufficiently wide, such that the use of optical fibers becomes morepractical in an aerospace application. Then, the temperatures of theoptical devices being controlled on the mobile platform can be allowedto drift, since because of the larger spacing between bands, the signalscannot cross into each others' bands. Furthermore, if sufficiently largebandwidth channels are employed, then some signals will always passthrough their designated channels, even when the bands (i.e., channels)on a transmitter and those of the other optical component receiving theoptical signals, such as a router, do not accurately align.

The drawback with the above described approach is that even in a typicalaerospace application in which the transmitter and receiver are locatedtogether, so that they are exposed to the same ambient temperature, theoptical devices that they communicate with, such as optical routers, aretypically located remotely from the receiver/transmitter. As a result,the remotely located optical routers are likely to be exposed to, andtherefore operating at, different temperatures from thereceiver/transmitter. In aerospace applications, this difference intemperature can be significant. The large temperature range thatvarious, remotely located optical devices may be exposed to can causelarge wavelength drifts in the input filters used with such devices. Foran optical based system to work with large temperature drifts, thewavelength bands must be so wide and so widely spaced apart that only avery limited (i.e., insufficient) number of bands can be fit into auseable optical spectrum on a given optical fiber.

Thus, it would be highly desirable to provide some means forcompensating for thermal drift in optical components, such as opticalrouters, employed on a mobile platform where the optical device can beexpected to experience significantly different thermal environments fromthose being experienced by a transmitter/receiver that is also beingcarried on the mobile platform. Accurately determining the thermal driftof the wavelength bands of the optical device and compensating for thethermal drift, without the need to control the temperature of theoptical device (i.e., allowing the temperature of the optical device to“float”), would allow a sufficiently large number of wavelength bands tobe implemented on a given optical medium to make use of an optical basedsystem more practical in aerospace and other applications.

SUMMARY OF THE INVENTION

The present invention is directed to a system and method for determiningthermal drift in an optical device located remotely from an opticaltransceiver and experiencing different thermal conditions than theoptical transceiver.

In one preferred form, the system involves using the optical transceiverto transmit a first pair of narrow band optical signals within a single,predetermined wavelength band to the optical device. The optical devicereceives a first pair of optical signals and generates a second pair ofnarrow band optical signals back to the transceiver over a suitableoptical medium. If the wavelength bands of the input of the opticaldevice have shifted due to thermal drift, at least one of the first pairof optical signals will be attenuated when it is received back at thetransceiver. The degree of attenuation, as well as which one of the pairof optical signals is attenuated more than the other, can be used todetermine the direction and magnitude of thermal drift of all of thewavelength bands at the remotely located optical device.

In another implementation of the present invention, the above-describeddetection of thermal drift of the wavelength bands at the optical deviceis corrected by the use of a thermal subsystem. The thermal subsystemcontrollably cools or heats the optical transceiver as needed to inducea controlled shift of the wavelength bands of the transmitter,representing the independent channels over which optical signals aretransmitted from the transmitter of the optical transceiver. In thismanner a controlled degree of thermal drift can be introduced into theoperation of the transmitter of the optical transceiver such that thecenters of the wavelength bands over which optical signals aretransmitted from the transmitter effectively “track” the centers of thewavelength bands at the input filter of the optical device.

In another preferred embodiment of the present invention RF signals areimpressed onto the optical signals transmitted from the transmitter tothe optical device. The optical device then transmits the opticalsignals with the RF signals impressed thereon to an optical receiver.The use of RF signals impressed on the optical signals enables opticalsignals that have shifted into adjacent passbands, as a result oftemperature differences between the optical device and the opticalreceiver, to be readily detected and cancelled at the receiver.Preferably, different frequencies are used for selected ones of theoptical signals such that optical signals transmitted on adjacentpassbands have different frequencies impressed on each.

In another alternative embodiment a filter media is used at an input ofthe receiver to shift the wavelength bands to which the receiver issensitive to match the wavelengths of the optical signals beingtransmitted from the optical device to an optical receiver. The opticalmedia, in one preferred form, may comprise a filter. The optical mediamay be physically moved so that its orientation relative to the input ofthe receiver, is altered to cause the needed degree of wavelengthshifting. Alternatively, the filter media may be deformed, such as bystretching or compression, to achieve the needed degree of shifting ofthe optical signals being received at the input of the receiver.

In still another alternative preferred embodiment a greater plurality ofreceive channels for the optical receiver are implemented than thenumber of optical channels used for optical signals transmitted by theoptical transmitter. The number of receive channels may comprise anymultiple of transmitter channels. The receive channels are furtherarranged such that no bandwidth gaps (i.e., no “dead bands”) are presentbetween adjacent receive channels. The use of multiple receive channelsfor each transmit channel enables each optical signal to be stronglydetected at the optical receiver and thus compensates for the reductionin magnitude of the received optical signals when the passbands of theoptical device and the optical receiver are misaligned.

Compensating for the thermal drift in the wavelength bands of theoptical device by introducing a controlled degree of wavelength bandshifting at the optical transceiver allows the wavelength bands to benarrower and spaced more closely together. This allows a greater numberof independent optical channels to be implemented on a given opticalmedium, such as on a given optical fiber, without the need to controlthe temperature of the remotely located optical device. The system andmethod of the present invention also allows an optical based controlnetwork to be implemented on a mobile platform which allows a reductionin cost, weight and volume of the components used to implement a controlnetwork on a mobile platform. In addition, the use of an optical basedsystem eliminates the EMI issues that would otherwise need to beaddressed and compensated for in various mobile platformimplementations.

The features, functions, and advantages can be achieved independently invarious embodiments of the present inventions or may be combined in yetother embodiments.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will become more fully understood from thedetailed description and the accompanying drawings, wherein:

FIG. 1 is a simplified block diagram of a preferred embodiment of thesystem of the present invention;

FIG. 2 is a diagram of signals being transmitted between an opticaltransmitter and an optical receiver via a remotely located opticalrouter, illustrating alignment of the passbands of the router when therouter is at the same temperature as the receiver and transmitter;

FIG. 3 is a diagram illustrating misalignment of the passbands of theoptical router and the result of this misalignment on the opticalsignals being received at the receiver;

FIG. 4 is a simplified block diagram of an alternative preferredembodiment of the present invention incorporating a subsystem formodulating RF signals onto the optical signals transmitted from thetransmitter to the optical router and a subsystem for detecting the RFmodulated optical signals at the receiver;

FIG. 5 is a waveform diagram of the optical signals having RF signalsimpressed thereon, and further illustrating when the passbands of theoptical router are aligned with the passbands of the optical receiver;

FIG. 6 is a graph illustrating the effect on the optical signals whenthe optical receiver passbands are misaligned with the optical routerpassbands;

FIG. 7 is yet another alternative preferred embodiment of the presentinvention incorporating a filter positioned at an input of the opticalreceiver for controllably shifting the wavelength bands of the opticalsignals being received by the receiver to compensate for thermaldrifting of the passbands of the optical router; and

FIG. 8 is a graph illustrating yet another alternative preferredimplementation of the present invention in which a greater number ofreceive channels than transmit channels are used for receiving theoptical signals at the optical receiver.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The following description of the preferred embodiment(s) is merelyexemplary in nature and is in no way intended to limit the invention,its application, or uses.

Referring to FIG. 1, there is shown a system 10 in accordance with apreferred embodiment of the present invention. While the system 10 isshown as being implemented on aircraft 12, it will be appreciated thatthe system 10 could be implemented on any mobile platform where anoptical control network is desired. The system 10 could also be employedin a fixed structure where an optical system is desired, and where anoptical control device of the network is to be located in an ambientenvironment that causes the optical device to experience significanttemperature changes that differ from those experienced by an opticaltransmitter.

The system 10, in this example, includes an optical device 14 in theform of an optical router coupled via a pair of optical fibers 16 and 18with an optical transceiver 20. The optical transceiver 20 includes anoptical transmitter 22 and an optical receiver 24 located within acommon housing or enclosure 26. The optical transceiver 20 is inbi-directional communication with a control subsystem 28. The opticaltransceiver 20 is also in communication with a heating/cooling subsystem30. The control subsystem 28 is further in bi-directional communicationwith the heating/cooling subsystem 30. The control subsystem 28 can bean independent subsystem as shown in FIG. 1 or it can be integrated intothe transmitter 22.

While only two optical fibers 16 and 18 have been illustrated in FIG. 1,it will be appreciated that typically a larger number, for example, 4-8optical fibers will be coupled between the optical transceiver 20 andthe optical router 14. Also, while optical fibers 16 and 18 have beenused for explanation purposes, other optical media such as free space,photonic bandgap fibers, or photonic crystals could be employed in lieuof optical fibers. Also, while an optical router will be used forpurposes of explanation, it will be appreciated that the teachings ofthe present application could be used with any form of opticalmultiplexer, de-multiplexer, “add-drop”, or other component wherechanging operating temperature of the device will cause thermal driftingof it optical channels.

With the present system 10, wavelength division multiplexing (WDM) isused to transmit a plurality of narrow band optical signals 32 overoptical fibers 16 from the optical transmitter 22 to the optical router14. The optical router 14 routes the signals as needed to variousaircraft subsystems which are typically clustered together in groups,for example, groups of 12 to 16 components. This will be furtherexplained in the following paragraphs.

The optical router 14 typically includes a filter having a plurality ofwavelength bands which may also be termed “passbands”. The opticaldevice 14 receives each one of the optical signals 32 within a specificpassband and returns an optical signal 34 over a corresponding passbandto the optical receiver 24. Thus, each passband forms a separate opticalchannel. Due to the fact that the optical transceiver 20 is located in adifferent location in the mobile platform 12 than the optical router 14,the router 14 will often be experiencing a different ambient temperaturethan the optical transceiver 20. If the optical router 14 is warmer thanthe optical transceiver 20, than the router will shift all of itspassbands in one direction, but if it is lower in temperature than theoptical transceiver, than the optical router 14 will shift all of itspassbands in the opposite direction. The control system 28 is used tomonitor the received signals 34 and to shift the transmitted signals 32so that the wavelength centers of each of the signals 32 match thecenters of the passbands of the optical router 14. The control system 28does this by controlling the heating/cooling subsystem 30 to either heator cool the transmitter portion 22 of the optical transceiver 20 asneeded to cause shifting of the optical signals 32 such that theirwavelength centers remain centered within the passbands of the opticalrouter 14. In this manner, the system 10 essentially forms a “closedloop” end system in which the received optical signals 34 are monitoredand the temperature of the optical transceiver 20 is continuouslycontrolled, in real time, via the heating/cooling subsystem 30. Thisenables the optical signals 32 to “track” the shifting passbands of theoptical router 14.

Referring now to FIG. 2, the operation of the system 10 will be furtherdescribed in connection with a simplified diagram of the passbands 36used by the optical router 14. In this diagram, the optical transmitter22 and the optical receiver 24 are illustrated on opposite sides of theoptical router 14 merely to aid in explaining the alignment of theoptical signals 32 and 34 with the passbands 36. However, thetransmitter 22 and receiver 24 are actually located together in housing26, as shown in FIG. 1.

By way of background, it will be understood that a typical opticalfilter shifts its passband by about 0.026 nm per degree C. Over a 180°C. range from −55° C. to +125° C. range, the passbands of the routershift by about 4.7 nm. Without the thermal drift compensation of thepresent invention this large of a passband shift would require passbandsthat are about 8 nm wide and on 10.0 nm centers. However, 10 nm spacingof 8 nm bands means that 48 nm of wavelength are required to carry justfour optical signals. But four signals (i.e., channels) per opticalfiber is insufficient to be beneficial from a cost/benefit standpoint inaerospace applications. Furthermore, if these signals needed to beamplified, two separate amplifiers would be required because present dayamplifiers have a useable gain bandwidth of only about 40 nm.

Temperature adjustment of the transmitter 22 is made practical by thefact that many lasers and tuned optical receivers have a wavelengthdrift with temperature of approximately 0.4 nm per degree C. This ismuch larger than the 0.026 nm per degree C. drift of the passbandfilters used in the optical router 14. This means that while the opticalrouter 14 may experience temperature variations over a 180° C. range,which produce 4.7 nm wavelength shifts, the transceiver 20 can track thewavelength shifts by having its temperature adjusted over only a 4.7nm/0.4 nm/° C.=11.75° C. range.

Thus, using the heating/cooling subsystem 30 to control the opticaltransceiver 20 over only an 11.75° C. range enables the wavelengthcenters of the optical signals to be shifted as needed to compensate forthe much larger temperature variation that the optical router 14experiences.

With further reference to FIG. 2, detecting the thermal drift of thepassbands of the router 14 is accomplished by generating two narrow bandoptical signals as a “first” or “temperature probe” optical signal 32 b₁, 32 b ₂. The first optical signal pair 32 b ₁, 32 b ₂ is centeredwithin a passband 36 b of the optical router 14, which can be viewed as“channel B”. Optical signals 32 a and 32 c are transmitted in passbands36 a and 36 c respectively (channels A and C). The passbands 36 a-36 c,in this example, are illustrated as having 4 nm wavelength bandspositioned on 5 nm spacing. If the passbands 36 a-36 c of the router 14have their centers aligned with the optical signals 32 a-32 c, then themagnitude of the signals 34 a-34 c transmitted back by the router 14,and received by the optical receiver 24, will each be approximately thesame magnitude as their corresponding signals 32 a-32 c.

Referring to FIG. 3, if the passbands 36 a have shifted because of theoptical router 14 being at a different temperature from the opticaltransceiver 20, then the centers of the passbands 36 a will be shiftedfrom the wavelength centers of the optical signals 32 a-32 c. In thisinstance, one or the other of the first pair of optical signals 32 b ₁and 32 b ₂ will be attenuated when it is received by the optical router14. In this example, since the passbands 36 a-36 c have shifted upwardlyin wavelength (downwardly in frequency), optical signal 32 b ₁ will beattenuated significantly as compared to signal 32 b ₂ because of beingcutoff by the rolloff of passband 36 b. This is due in part to the factthat the passbands 36 a-36 c do not have perfectly flat tops, but ratherrounded tops. By using the pair of optical signals 32 b ₁ and 32 b ₂ andspacing these narrow band optical signals both within a 4 nm wavelengthband, the attenuation of one or other can be detected. It will also benoted that signals 32 a and 34 c are also slightly reduced in intensityas a result of no longer being centered at the wavelength centers of thepassbands 36 a and 36 c, respectively.

The control subsystem 28 monitors the second pair of signals 34 b ₁ and34 b ₂ sent by the optical router 14 and from the degree of attenuationof one or the other of this pair of signals, as well as which one of thepair is attenuated, determines the degree of thermal shift of thepassbands 36 a-36 c of the optical router 14. The control subsystem 28causes the heating/cooling subsystem 30 to heat or cool the transmitter22 as needed to apply a real time correction to the narrow band lasersused in the optical transmitter 22. The correction shifts the signals 32a-32 c either higher or lower along the wavelength spectrum so that thecenters of the optical signals 32 a-32 c are centered with the passbands36 a-36 c of the optical router 14.

Thus, the system 10 does not attempt to control the temperature of theoptical router 14, but instead focuses on controlling the temperature ofthe optical transmitter 22 in a manner that induces a controlled amountof thermal shifting of the signals 32 a-32 c from the transmitter asneeded to match the thermal shifting of the passbands 36 a-36 c. Thisenables the received signals 34 a-34 c received by the optical receiver24 to be maintained as strong in magnitude as possible.

The second significant benefit of this approach is that the passbands 36a-36 c can each be made much narrower. This enables wavelength divisionmultiplexing (WDM) of optical signals to be made practical for aerospaceapplications where devices needing separate wavelengths are usuallyclustered in groups (typically of 12 to 16). As one specific example,one motor in an aerospace electrical actuator may send 12 separateoptical sensor signals to a flight control computer. Without thetemperature compensation of the present invention, wavelength divisionmultiplexing channels of an optical router would need to be spaced 10 nmapart and only 4 signals could be put into a single optical fiber.Consequently, 6 fibers (3 excitation fibers, where each carries 4excitation signals to the sensors via the router, and 3 fibers bringingthe modulated signals back via the router) would need to be used toexcite and collect signals from the 12 sensors. However, if thepassbands can be put on 3.3 nm centers, then one fiber may carry 12signals and only 2 (1 fiber going out and one coming back) would beneeded for actuator motor sensing in this example. Reducing the totaloptical fiber count becomes especially important at a flight controlcomputer because if a vehicle has a dozen flight control surfaces therewill typically be 12 actuators (one for each flight control surface). Ifeach actuator has two motors and if there are 4 wavelengths in a fiber,there would need to be 144 fibers (12 actuators×2 motors per actuator×6fibers per motor), just for sensing purposes alone, that will needconnector space on the face of the flight control computer. With 12wavelengths in a single fiber, there would only need to be 48 fibersconnected to the face of the flight control computer.

The system and method 10 of the present invention thus allows the use ofan optical network to be implemented on a mobile platform withsignificantly fewer optical fibers being required for a givenimplementation. The system and method 10 further eliminates concernswith EMI that would otherwise be present with fly-by-wire controlsystems. The system and method 10 further reduces the mass, complexityand cost of an optical control network by its ability to “squeeze” moreoptical channels onto a given optical fiber without the risk ofperformance degradation that would be otherwise incurred from thethermal drift experienced by the optical router 14.

Referring to FIG. 4, an alternative preferred implementation of thepresent invention is illustrated and designated by reference numeral100. Components in common with system 10 have been labeled withcorresponding reference numerals having a prime (′) symbol. With thesystem 100, the optical transmitter 22′ and the optical receiver 24′ areno longer located within a common housing or enclosure, but are insteadlocated remote from each other. In addition, the optical device 14′ islocated remotely from the optical transmitter 22′ and the opticalreceiver 24′. Since the optical transmitter 22′ and optical receiver 24′are not located in a common enclosure, they will not necessarilyexperience the same ambient temperature, and thus may not be at the sameoperating temperature. Thus, the passbands at the output of the opticaldevice 14′ may not be aligned with the passbands at the input of theoptical receiver 24′, and the passbands at the input of the opticalreceiver 24′ will not necessarily track the wavelength centers of thetransmitted signals from the optical transmitter 22′. In this instancethe thermal drift of the optical receiver 24′ cannot simply becontrolled by heating/cooling it via the heating/cooling subsystem 30′because the optical transmitter 22′ and the optical receiver 24′ arelocated remotely from each other. Moreover, the optical receiver 24′ maybe located in an avionics bay or at some other area where it would beundesirable to attempt to heat the optical receiver 24′. Thus, analternative system is needed for detecting when the optical device 14′is operating at a different temperature from the optical receiver 24′,and thus introducing misalignment of the passbands at the output of theoptical device 14′ with the passbands of an input filter of the opticalreceiver 24′. This embodiment accomplishes detection of this router14′/receiver 24′ passband misalignment by impressing a unique (i.e.,different) radio frequency (RF) modulation signal from an RF modulationsubsystem 40 onto each of the optical signals 32 a, 32 b and 32 c. Aprocessor 42 is used in connection with the optical receiver 24′ todetect when an RF modulated optical signal is being received in apassband that it should not be received in, thus indicating misalignmentof the two passbands.

A variety of different types of signals can be readily impressed on thecontinuous wave outputs of the narrowband lasers used to form theoptical signals 32. For example, sinusoidal excitation signals such asAM, FM and Phase Modulation (PM) could be employed. Digital signalscould also be modulated onto the optical signals 32 a-32 c, andseparated from each other by alternating them with analog signals. Ineither event, the RF modulations allow positive identification of eachof the optical signals 32 a-32 c, even in the event thattemperature-induced wavelength drift of the receiver 24′ causes a signalfrom one channel of the router 14′ to cross into an adjacent channel atthe receiver 14′ input.

Referring to FIG. 5, an example of the above-described modulation schemeis illustrated. An 810 KHz signal is impressed on optical signal 32 a ₁.A 600 KHz signal is impressed on optical signal 32 a ₂. A 1.0 MHz signalis impressed on optical signal 32 b, and a 2.1 MHz excitation signal isimpressed on signal 32 c. For convenience, optical signals 32 a ₁ and 32a ₂ can be viewed as being placed on channel “A”. Optical signal 32 bcan be viewed as being placed on an optical channel “B”, and opticalsignal 32 c on channel “C”. The passbands of the router 14′ aredesignated by waveform 44. The passbands of the receiver 24′ aredesignated by waveform 46. The specific frequencies above are merelyexemplary and may be varied.

When the passbands 44 of the optical device 14′ are aligned with thewavelength centers of the signals 32 generated by the transmitter 22′,the modulated signals appear as indicated by reference numeral 48 inFIG. 5. The optical signals 32 a-32 c are centered within theirrespective passbands 44. Since the receiver 24 passbands 46 are alignedwith the router passbands 44, the signals received by the opticalreceiver appear as indicated by reference numeral 50. The RF modulatedoptical signals 32 a-32 c remain centered within the receiver passbands46 and are substantially of the same magnitudes as indicated byreference numeral 48. It will be noted, however, that the RF modulatedoptical signals 32 a-32 c, as indicated by reference numeral 48, arereduced slightly in magnitude, as compared to the signals 32 a-32 coutput by the transmitter 22′.

Referring now to FIG. 6, the effect on the RF modulated optical signals32 a-32 c can be seen when the router passbands 44 do not align with thepassbands 46 of the optical receiver 24′. The RF modulated outputsignals 32 a-32 c, as indicated by reference numeral 48, are stillaligned with the router passbands 44. However, due to the shift of thereceiver passbands 46, the RF modulated optical signals 32 a-32 c nowhave drifted or “bled” into adjacent channels. Specifically, a portionof signal 32 a ₂ has now drifted into channel B, and a portion of signal32 b has drifted into channel C. In this example, the optical receiver24′ may be located in an avionics bay, near the transmitter 22′, andwill therefore not be as hot as the filters in the optical router 14′,which are located in a considerably warmer, remote location of themobile platform 12.

Modulating RF signals of different frequencies onto the optical signals32 a-32 c allows the processor 42 (FIG. 4) to detect when an unwantedsignal is present within a given optical channel and to cancel (i.e.,reject) that portion of the signal within a given channel. Thus,processor 42 rejects that portion of optical signal 32 a ₂ that ispresent within channel B in FIG. 6. Similarly, the processor 42 rejectsthat portion of optical signal 32 b that is present within channel C.

Since the signals received by the optical receiver 24′ in FIG. 6 havealso been attenuated significantly, the receiver 24′ also makes amagnitude compensation to the received signals designated by referencenumeral 50 in FIG. 6. One way of achieving this compensation is bydirectly measuring a temperature of the optical receiver 14′ using asuitable temperature sensor 42 a in communication with the processor 42.If the characteristics of the filter of the optical receiver 24′ areknown, then the difference in temperature between the optical router 14′and the optical receiver 24′ can be determined. A suitable look-up tablecan be used which includes magnitude correction values dependent uponthe temperature to correct for the signal attenuation caused by themismatch of the receiver passbands

Referring to FIG. 7, another alternative preferred embodiment of thepresent invention is illustrated and represented by reference numeral200. This approach does not make use of impressing RF modulation signalsonto the optical signals 32 a-32 c as described in connection with FIGS.4-6, but instead uses a mechanically adjustable input filter 202 toshift the passbands 46 at an input side of the optical receiver 24′. Themechanically adjustable input filter 202 is controlled by a controller204 that monitors operation of the optical receiver 24′, and moreparticularly its temperature, and uses the temperature information toapply suitable control signals to alter the mechanically adjustableinput filter 202 as needed to achieve the needed degree of passband 46shifting. The mechanically adjustable input filter 202 may comprisedevices as diverse as Bragg gratings, Fabry-Perot etalons, SurfaceAcoustic Wave devices, and Micro-Electro-Mechanical Machine gratings.The mechanically adjustable input filter 202 may be physically alteredin position relative to the optical receiver 24′ to achieve the neededdegree of shifting of the passbands 46. Alternatively, it may bephysically deformed such as by mechanical stretching or compression,which also will cause the needed shifting of the passbands 46. In thisimplementation the controller 204 also makes use of a look-up tablewhich includes information correlating the temperatures of the opticaltransmitter and receiver 24′, and the transmission characteristics ofthe filters in the optical device 14′ and the receiver. With thatinformation, it is possible for the controller to determine thewavelength shift of the device 14′ and alter the emission wavelengths ofthe transmitter and the passbands of the receiver 24′ to center eachsignal in each passband of the optical device 14′.

Referring to FIG. 8, an alternative implementation of the system 100described in connection with FIGS. 4-6 will now be described thatinvolves the use of a greater number of receive passbands (i.e.,channels) 46 than router passbands 44. In this example, there are tworeceive passbands 46 a assigned for channel A, two passbands 46 bassigned for channel B and two passbands 46 c assigned for channel C.Thus, there are two receive passbands for each one of the routerpassbands (i.e., channels) 44. The use of multiple receive channels foreach channel of the optical router 14′ is beneficial because of thesignal attenuation that occurs when the receive passbands 46 aremisaligned with the router passbands 44. The optical receiver 24′ stillneeds to make sense of the signals present within each of the passbands46. However, instead of using analytic compensation as described inconnection with the system 100 of FIGS. 4-6, the use of multiple receivechannels for each passband of the router 14′ allows the signals 32present within each passband of the optical router 14′ to be stronglydetected by the optical receiver 24′. This approach also avoids the needto change the gain of the receive amplifiers used within the opticalreceiver 24′, which would otherwise be needed to compensate for the weakoptical signals 32 a-32 c received by the optical receiver 24′, but atthe cost of increased noise in the receiver 24′. Since there are no“deadbands” or “gaps” between the receiver passbands 46 where a signal(or portion of a signal) may be lost, the signals 32 a-32 c can bedetected more readily. While two receive channels are shown for eachrouter passband, it will be appreciated that any multiple of receivechannels could be employed. Thus, three, four or more receive passbandsper router passband could be employed. From a practical standpoint,however, the maximum number of receive passbands that may be employedper router passband will depend largely on the bandwidth of each routerpassband.

With the multiple receive channel approach described in FIG. 8, thetemperature of the transmitter 22′ is still controlled as described inconnection with the system of FIG. 4, however, no shifting of thepassbands 46 of the receiver 24′ is employed. When using multiplereceive channels, however, the temperature of the optical router 14′will be sensed in two ways. The first is to compare the ratio of the twosignals that appears in only the first passband (channel A). This is themethod identified previously. In practice, it is only reliable if thedrift of the passband with respect to the two signals is so small thatthough one signal is in the roll-off region of the passband, and so issmaller than the signal near the center of the passband, the smallersignal is still fairly strong, such as greater than 50% of the strengthof the strong signal. If the drift of the passbands is such that onesignal is near the extreme edge of the passband, its strength at thedetector in the receiver may be close to the noise level of thedetection system. That means that the strength of the weak signal ismore uncertain than the strength of the strong signal. Dividing theuncertain, weak signal by the certain, strong signal to obtain theratio, and hence, the temperature, gives a value with the uncertainty ofthe weak signal. The second approach makes use of the large number ofpassbands of the present receiver 24′. It is useful for the case whendrift of the passbands is large. In that case, rather than use the veryweak signal that is passed by the filter in the first passband, it isbetter to use the stronger version of the same signal that is passed bythe filter in the adjacent passband. Thus to obtain an accuratetemperature for the router 14′ if its temperature-induced drift issmall, the ratio of the two optical signals 32 a ₁ and 32 b ₁ appearingin the first passband (channel A) is used, but for large drifts, thestrength of the signal 32 a ₁ in the first passband (channel A) iscompared with the strength of the signal 32 a ₂ in the second (adjacent)passband (channel B).

While various preferred embodiments have been described, those skilledin the art will recognize modifications or variations which might bemade without departing from the inventive concept. The examplesillustrate the invention and are not intended to limit it. Therefore,the description and claims should be interpreted liberally with onlysuch limitation as is necessary in view of the pertinent prior art.

1. A method for forming a closed loop temperature monitoring and controlsystem in an optical system, the method comprising: transmitting anoptical temperature probe signal having an initial, predeterminedmagnitude over an optical medium to an optical device, the opticaldevice having at least one passband forming an optical channel;receiving the optical temperature probe signal in said passband at theoptical device; using the optical device to transmit said opticaltemperature probe signal back to a receiver; analyzing said opticaltemperature probe signal received by said receiver to determine if amagnitude of said optical temperature probe signal has changed from saidinitial, predetermined magnitude; using a detected change in saidinitial, predetermined magnitude to determine a degree of thermal shiftof said passband; and modifying the temperature of a transmittertransmitting said optical temperature probe signal so that asubsequently generated temperature probe signal is positioned withinsaid passband.
 2. The method of claim 1, further comprising forming saidoptical temperature probe signal as a pair of independent opticalsignals having the same magnitude.
 3. The method of claim 2, whereindetermining if said change in said initial, predetermined magnitude ofsaid optical temperature probe signal has occurred comprises determiningif one of said pair of independent optical signals is greater than theother.
 4. The method of claim 3, further comprising using a differencein magnitude of said independent pair of optical signals to determine ifsaid passband has increased or decreased in frequency as a result ofthermal shifting of said passband.
 5. The method of claim 4, furthercomprising analyzing said optical temperature probe signal in real timeand modifying said temperature of said transmitter in real time.
 6. Amethod for real time monitoring and compensation of thermal drift of anoptical device receiving optical signals from an optical transceiver,wherein the optical device has a plurality of adjacent passbands, themethod comprising: generating a pair of optical signals, each one beingof a predetermined magnitude and being sufficiently narrow so that bothoptical signals fit within a predetermined bandwidth, and transmittingsaid pair of signals over an optical medium in accordance with one ofsaid passbands of said optical device that represents a referencepassband, from said optical transceiver to said optical device; usingsaid optical device to receive said pair of optical signals over saidoptical medium and to transmit said optical signals back over a returnoptical medium back to said transceiver; using said transceiver toreceive said second pair of optical signals from said optical device;comparing said optical signals with one another; and from saidcomparison, determining a needed operating temperature change of saidtransceiver to cause a bandwidth shift of said pair of said opticalsignals when said pair of optical signals are transmitted from saidoptical transceiver that maintains said optical signals positionedwithin said reference passband.
 7. The method of claim 6, furthercomprising the step of using a thermal system to at least one of heatand cool said transceiver as needed to affect operation of saidtransceiver in a manner that causes said transceiver to generate saidfirst pair of optical signals such that a bandwidth within which saidpair of optical signals is located is aligned with said referencepassband.
 8. The method of claim 6, wherein comparing said pair ofoptical signals comprises comparing magnitudes of said pair of opticalsignals to determine if a shift of said reference passband has occurredfrom a predetermined bandwidth.
 9. The method of claim 7, furthercomprising forming a closed loop system to at least periodically performsaid comparing of said first and second optical signals andautomatically, in real time, adjust said operating temperature of saidoptical transceiver to maintain said bandwidth of said pair of opticalsignals aligned with said reference passband.
 10. The method of claim 9,further comprising, from said difference in magnitude between said pairof optical signals, which direction said reference passband has shifted.11. The method of claim 10, further comprising applying a magnitudecompensation to said received optical signals if needed.
 12. A closedloop optical communication system for use with an optical device havinga plurality of input channels affected by changes in operatingtemperature, the system comprising: an optical transmitter fortransmitting an optical temperature probe signal within a predeterminedbandwidth to one of said input channels of the optical device; anoptical receiver for receiving the optical temperature probe signal backfrom the optical device; a controller for monitoring a characteristic ofsaid optical temperature probe signal received by said optical receiverand using a change in said characteristic to determine if a bandwidth ofsaid one input channel of said optical device has shifted due to achange in operating temperature of said optical device; and atemperature subsystem responsive to said controller for at least one ofcontrollably heating and cooling said optical transmitter to induce athermal shift in said optical temperature probe signal needed tomaintain said optical temperature probe signal approximately centeredwithin said bandwidth of said one input channel of said optical device.13. The system of claim 12, wherein said temperature probe signalcomprises a pair of optical signals sufficiently narrow in bandwidth toboth fit within said predetermined bandwidth.
 14. The system of claim12, wherein said characteristic of said optical temperature probe signalcomprises a magnitude of said optical temperature probe signal.
 15. Thesystem of claim 14, wherein said optical temperature probe signalcomprises a pair of optical signals; and wherein said controllercompares said pair of optical signals in magnitude to determine if amagnitude difference exits, in order to determine that a thermal shiftof said bandwidth of said input channel has occurred.
 16. The system ofclaim 12, wherein said transmitter transmits a plurality of independentoptical signals to said optical device for reception in said inputchannels of said optical device; and wherein said controller causes abandwidth of each of said independent optical signals to be shifted inaccordance with said shifting of said optical temperature probe signalto compensate for thermal drifting of said input channels of saidoptical device.
 18. A closed loop optical communication and monitoringsystem for use with an optical device, wherein the optical deviceincludes a filter having a plurality of passbands defining independentoptical reception channels, the passbands being affected by an operatingtemperature of the optical device, the system comprising: a transceiverfor transmitting a plurality of independent optical signals over acorresponding plurality of optical channels each defined by apredetermined bandwidth, the transceiver being in two way communicationwith said optical device; said transceiver operating to transmit a firstpair of optical signals each having a predetermined bandwidth over adesignated one of said optical channels to a reference passband of saidfilter, and to receive a second pair of optical signals back from saidoptical device corresponding to said first pair of optical signals; amonitoring subsystem for monitoring said second pair of optical signalstransmitted from said optical device back to said transceiver and forcomparing said first and second pairs of optical signals, and from saidcomparison determining if said passbands of said filter are aligned withsaid optical channels of said transceiver; and a temperature controlsubsystem responsive to said monitoring subsystem for at least one ofheating and cooling said transceiver as needed to shift said first pairof optical signals as needed to maintain said first pair of opticalsignals positioned within said designated one optical channel.
 19. Thesystem of claim 18, wherein said monitoring system uses said comparisonto determine which one of said pair of second optical signals is largerin magnitude, and to determine therefrom whether said passbands haveshifted either upwardly or downwardly in frequency.
 20. The system ofclaim 18, wherein said temperature control subsystem comprises both aheating unit and a cooling unit.